Heavy duty electrical machinery used in industrial settings is generally driven by inductive motors. Typically, the motors are rated to operate at particular current ranges.
Overload relays are typically employed to protect such expensive machinery from damage due to overheating resulting from excessive current flow through the motors. An overload relay is merely a sensor that is connected between the power supply and the load and upon the detection of a current overload condition, disconnects the load from the power supply.
An overload relay is usually designed to operate over a wide range of values and the user must set the trip current based upon the specifications of the motor in use. The trip current defines the value at which the relay is triggered into breaking the circuit between the load and the power. The relay's internal current overload detection circuit generally includes a linear taper potentiometer that can be adjusted to set the relay trip current. The current setting is a reciprocal function of the potentiometer resistance so that as the potentiometer resistance is increased, the relay trip current value is decreased and as the potentiometer resistance is decreased, the relay trip current value is increased. As a result, the potentiometer dial markings for the trip current settings get closer and closer together as the value of the current increases. FIG. 1 is an illustration of a known dial markings for setting the trip current. FIG. 2 depicts the relationship between dial rotation in degrees versus the dial setting in amperes. The graph demonstrates how at the higher current settings a smaller degree of rotation of the dial creates a greater change in the current setting as opposed to the lower current range where a smaller degree rotation of the dial creates a smaller change in current setting. This makes it more difficult for the user to accurately select trip current values at the higher end of the scale. Inaccuracies in setting the trip current can result in extensive damage to expensive machinery.
Clearly, it would be desirable to use an overload rely where the dial markings would be sufficiently spaced to enable a more accurate selection of the relay trip current at higher ampere values. The protection afforded by a more precise overload relay would enable to the user to operate heavy equipment closer to its optimal performance levels without risk of expensive damage. The present invention seeks to achieve these objectives.